Latent errors pose the greatest threat to safety in a complex system because they are often unrecognized and can result in multiple types of active errors. Analysis of the Challenger accident traced contributing events back nine years. Latent errors can be difficult for the people working in the system to notice as the errors may be hidden in computer program design of routine processes or in the structure or management of the organization. People also become accustomed to design defects and learn to work around them, so they are often not recognized.